I stumbled into the same question last year, when doing my SysIII to RV64 port. I managed to turn that into a somewhat chaotic discussion, mixing old and new, and history with ideas. From that chaotic discussion I got the impression that it was indeed mostly ad hoc. In context, hardware was much easier to boot and drive back then -- it probably was not seen as complex enough to warrant much research into layering and abstraction. Also bear in mind that something like a boot rom only became the norm in the late 70’s. Before that, one keyed in two dozen words with a tiny program to load the first boot stage. That said, there is an implicit layering in v7 and beyond: - “low.s" does hardware setup, incl. such stuff as setting up interrupt tables. As this is closely tied to the hardware, it would have been a custom job in each case. - “mch.s” (later also mch.c) has the basic routines that are hardware dependent (switching stacks, changing priority levels and modes, etc.). It also has emulation for ‘missing’ instructions, such as floating point ops where this is not available in hardware. Same as above, I think. Maybe h/w related memory protection operations should live here as well, but the hardware was still quite divergent in this area in the 70’s and early 80’s. - low-level device drivers live in the ‘dmr’ or (later) ‘io’ directory. Here there is some standardisation, as all device drivers must conform to the (char/block) device switch APIs. It seems to me that most of these drivers were written by taking one that was similar to what needed to be written and to start from there. Maybe this is still how it works in Linux today. - To the extent that there is such a thing as 'high-level device drivers’ in early Unix, the structure is less clearly visible. The file system (and there was only one at the time of v7) is placed between the block device switch and the mount table so to speak. This was structured enough that splicing in other file systems seems to have been fairly easy in the early 80’s (the splicing in, not the writing of the file system itself, of course). Starting with 8th edition, the ‘file system switch’ created a clear API for multiple file systems. Arguably, the ‘tty’ subsystem is also a ‘high-level device driver’, but this one lives as custom code together with the serial port device drivers. Also in 8th Edition, ‘streams' were introduced. One could think of this as a structured approach to high-level device drivers for character mode devices, incl. the ’tty’ subsystem. - I don’t think there was ever anything in early Unix that merged ’streams’ and the 'file system switch' into a single abstraction (but maybe 9P did?). I have a similar interest, but to avoid the same chaos as I created before, I’ll respond to this with a pm.